Production method of a sensor chip and computerized tomography detector

ABSTRACT

A sensor chip, in particular for computerized tomography detectors, including an analog-digital converter electrically connected to an element detecting radiation. A problem addressed is that of defining a sensor chip which is as cost-efficient and reliable as possible. According to an embodiment of the invention, only one single crystalline base plate is used, on which all required components of the sensor chip are applied. A through-contact between the conductor paths or the contacts of both sides of the base plate is used as applicable in order to connect the components of both sides to each other.

PRIORITY STATEMENT

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/EP2014/055488 which has anInternational filing date of Mar. 19, 2014, which designated the UnitedStates of America and which claims priority to German patent applicationnumber DE 102013206404.7 filed Apr. 11, 2013, the entire contents ofwhich are hereby incorporated herein by reference.

FIELD

At least one embodiment of the invention generally relates to a sensorchip, in particular for computerized tomography detectors, having ananalog-digital-converter electrically connected to an element detectingradiation. Furthermore, at least one embodiment of the inventiongenerally relates to a computerized tomography detector, a productionmethod of such a sensor chip, and/or a method for operating a sensorchip.

A sensor chip is generally understood to mean a sensor circuit, theelectrical and electronic components of which were applied onsemiconductor substrates or integrated therein. The semiconductorsubstrate is generally designated as a wafer or base plate which as awhole is produced with a multiplicity of sensor circuits in order tothen separate these from each other and wire them after the productionprocess.

BACKGROUND

Such sensor chips are used in computerized tomography detectors (CTdetectors) which are used for medical imaging. A computer tomographpermits better diagnosis if a high resolution, i.e. a high number ofpixels per surface, is ensured. One sensor chip usually corresponds toone pixel, whereby to achieve the desired resolution as a rule amultiplicity of sensor chips is required. Lithographic integration insemiconductor substrates is already advantageous for cost reduction onaccount of this multiplicity of pixels required.

In addition, the sensor chips must be suitable for arranging in adefined two-dimensional manner with additional sensor chips in order toprecisely define the pixel spacing. Each dimension requires the flatarrangement of two adjacent sensor chips. Consequently, it must bepossible to arrange a total of four sensor chips laterally. Especiallyfor this quadrilateral arrangement requirement, as is conventional forlarge-area CT detectors, sensors based on semiconductor substrates arealso highly suitable.

For example, DE 10 2007 022 197 A1 discloses a detector element for anX-ray detector for use in an X-ray computer tomograph. This detectorelement has a number of components based on semiconductor substrates,namely a detection element, a module with electronic circuits and acontact element which is arranged between the two aforementioned and hastwo functions, namely on the one hand to ensure mechanical stabilizationof the sensor element and on the other hand to adapt the geometricarrangement of the signal output contacts of the detection element tothe geometric arrangement of the signal input contacts of the module byway of its conductor paths.

A detector module for a radiation detector is known from DE 10 2010 011582 B4 which has the function of an optoelectrical converter layer withdownstream rewiring on the associated substrate, whereby on account ofthe novel arrangement of the signal output contacts the cable-basedconnection of the external readout electronics is easier.

The production and material costs are very high with the currentsolutions, particularly as a so-called wafer-bonding method often has tobe employed or the contact costs are simply very high.

SUMMARY

At least one embodiment of the invention improves the production processand/or reduces the material costs for sensor chips of the aforementionednature and positive properties, such as quality or service life.

Embodiments of the invention are directed to a sensor chip, acomputerized tomography detector, a production method of a sensor chipand a method for operating a sensor.

According to at least one embodiment of the invention a sensor chip, inparticular for computerized tomography detectors, has ananalog-digital-converter electrically connected to an element detectingradiation and a crystalline base plate, wherein components of theelement detecting radiation and components of the analog-digitalconverter are lithographically integrated on one detector side of thecrystalline base plate. This is extremely advantageous because twocrystalline base plates need not pass through the lithographicintegration process of the necessary circuits in order to then be linkedto each other in a bonding method in a likewise expensive manner.Furthermore, the sensor chip according to at least one embodiment of theinvention ensures very robust electronic circuitry which, especially inthe case of a multiplicity of sensor chips in CT detectors, helps thelatter to achieve more stable operation and a longer service life. Inaddition, a likewise advantageous optimization of the installation spaceis obtained.

An embodiment of the invention is directed to a production method of asensor chip including at least:

a first step where components of an element detecting radiation andcomponents of an analog-digital converter are lithographicallyintegrated on a detector side of a crystalline base plate,

a second step where the detector side is insulated with the componentsof the element detecting radiation and the components of theanalog-digital converter,

a third step where a through-contact for the electrically conductiveconnection of conductor paths of the detector side is embodied withconductor paths of the second side of the crystalline base plate, and

a fourth step where the second side of the crystalline base plate isinsulated.

An embodiment of the invention likewise comprises a method for theoperation of a sensor chip in which radiation is detected on onedetector side of a crystalline base plate of the sensor chip and thedetected radiation is converted into an analog signal and the analogsignal by way of an analog-digital converter into a digital signal.

Further advantageous embodiments and preferred developments of theinvention can be found in the description of the figures and/or thesubclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail hereinafter with reference tothe example embodiments shown in the figures.

The figures show:

FIG. 1'a sectional view of a first sensor chip in a production sectionbefore the through-contact,

FIG. 2—a sectional view of the first sensor chip from FIG. 1 after athrough-contact to a conductor path of the detector side,

FIG. 3—a sectional view of the first sensor chip from FIG. 2 after theapplication of a connection element embodied as a solder ball,

FIG. 4—a sectional view of a second sensor chip with aradiation-permeable base plate, in which all the components are arrangedon the detector side, and

FIG. 5—a sectional view of the second sensor chip from FIG. 4 afterinsulation of the detector side and application of the solder balls.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

According to at least one embodiment of the invention a sensor chip, inparticular for computerized tomography detectors, has ananalog-digital-converter electrically connected to an element detectingradiation and a crystalline base plate, wherein components of theelement detecting radiation and components of the analog-digitalconverter are lithographically integrated on one detector side of thecrystalline base plate. This is extremely advantageous because twocrystalline base plates need not pass through the lithographicintegration process of the necessary circuits in order to then be linkedto each other in a bonding method in a likewise expensive manner.Furthermore, the sensor chip according to at least one embodiment of theinvention ensures very robust electronic circuitry which, especially inthe case of a multiplicity of sensor chips in CT detectors, helps thelatter to achieve more stable operation and a longer service life. Inaddition, a likewise advantageous optimization of the installation spaceis obtained.

The element detecting radiation may be a photodiode or a photodiodearray, the various doped transitions of which are realized by way oflayer-by-layer semiconductor layer deposition based on lithographicmasks. In the process, the element detecting radiation can detect theanalysis radiation, in other words, in the case of a tomograph, thatradiation which has also been transmitted via the body of a living beingand consequently carries medical information. The element detectingradiation converts this analysis radiation into an analog electricalsignal so that the medical information contained therein can be passedon electrically.

Alternatively, indirect detection can be realized by the elementdetecting radiation, for example, scintillation light from ascintillation element converts the analysis radiation from the body intodifferent radiation, for example, visible light. The scintillation lightis detected in the element detecting radiation of the sensor chip andthere converted into an analog, electrical signal. The intermediate stepis therefore necessary because semiconductor detectors, as typicallyused in the sensor chip, are only sensitive in a particular wavelengthrange of electromagnetic radiation. If the wavelength of the radiationsource of the medical device, for example, an X-ray tomograph, liesoutside this range indirect detection must be pursued.

The analog-digital converter is designed to convert the analogelectrical output signal of the element detecting radiation into adigital signal. This circuit of the analog-digital converter islithographically applied on the crystalline base plate and uses amultiplicity of different semiconductor layers, conductor paths and/orother components of integrated circuits.

The crystalline base plate can be a monocrystalline base plate, in otherwords, a section of a so-called wafer. Alternatively, depending on theembodiment of the invention, a polycrystalline base plate can also beused. Typical materials of such a base plate are silicon, siliconcarbide, gallium arsenide or indium phosphide, each in monocrystallineor polycrystalline state. If the radiation to be detected is transmittedthrough the base plate, the spectral transmission properties of therespective material must also be considered in order to avoidunnecessary absorptions.

Lithographic integration is understood to mean the application of anapplication-specific, integrated circuit on the crystalline base plate.For this reason, the sensor chip may also be a so-calledapplication-specific integrated circuit (ASIC) which is realized as astandardized integrated, electronic circuit. The function of an ASICcannot be subsequently manipulated but the production costs aresignificantly lower compared to non-standardized integrated circuits.

A particular advantage of at least one embodiment of the invention isthe fact that the components of the element detecting radiation and theanalog-digital converter are both lithographically integrated on thefirst side of the crystalline base plate, namely the detector side.Thus, the costly integration process must only be performed once on thebase plate and all the electrical and electronic components can beproduced simultaneously and spatially in parallel and not, as hitherto,consecutively.

Advantageously, the analog-digital converter is electrically connectedto an electrical connecting element on a second side opposite thedetector side of the crystalline base plate via a through-contact of thecrystalline base plate. Such a through-contact between conductor pathson the opposing sides of the base plate can be achieved by way of anetching process which chemically removes the base plate at an uncoveredsite vertically with regard to the surfaces of both sides until aftersome 100 micrometers a conductor path or a contact especially providedfor this purpose is exposed. Insulation is followed by metal platingwhich creates an electrically conductive connection of the conductorpaths of both sides as it settles both on the exposed contact or theexposed conductor path, as well as on the internal lateral surfaces ofthe recess produced by etching, as well as on a conductor path or acontact of that side of the base plate from which etching was started.Thus, the former redistribution contacts can be reduced to a minimum ofmaterial and occupied space.

In an advantageous embodiment, the through-contact has metal platingwhich is provided for the transmission of a digital signal from thedetector side, through the crystalline base plate to the second side ofthe crystalline base plate. The digital signal is signal converted bythe analog-digital converter which is passed on to an analysis unitwhich can store, display and/or otherwise process this signal. Thedesign of a through-contact for digital signals is not as critical as,by comparison, for analog signals. The reason for this is that thedigital signal operates with detection thresholds for the values 0 and1, whereas any variation in voltage or current with an analog signaldirectly affects the information content of the signal. For this reason,with a through-contact less attention need be paid to leakage currentsor unwanted capacities in the case of digital signals.

The through-contact is at least partially formed by a recess, inparticular an etched recess, in the crystalline base plate. However,this through-contact may have further features. For example, the recessmay be filled with a dielectric again or alternative or additionalmechanical methods may have been used to produce the recess such as, forexample, a bore.

An electrical connecting element can be applied on both the detectorside and on the side of the crystalline base plate opposite the detectorside. It serves to produce as safe and simple a contact as possible withan analysis unit such as, for example, a computer. The connectingelement may be a solder ball, a printed circuit board contact, a socketor a plug, which are integrated into the base plate respectively or canbe electroconductively connected hereto.

In an advantageous embodiment, the crystalline base plate is permeablefor the radiation detectable by the element detecting radiation and theelectrical connecting element is likewise applied on the detector sideof the crystalline base plate. In order to further minimize theradiation losses in the base plate, these may be thinned in order toreduce the thickness of the base plate. No through-contacts arenecessary for this embodiment, particularly as all the components(analog-digital converter, element detecting radiation, connectingelement) are arranged on the detector side of the base plate and onlythe radiation to be detected is transmitted through the base plate. Thisis therefore a particularly cost-efficient embodiment.

The detector side is generally understood to be the side on which theelement detecting radiation is embodied. The detector side may be boththe front of the detector facing the radiation to be recorded as well asthe reverse of the detector averted from the radiation to be recorded.

The components of the element detecting radiation, the components of theanalog-digital converter and the components of the connecting elementare electrical or electronic components, in particular electricallyconductive conductor paths, undoped and/or doped semiconductor layers.Any elements known from semiconductor technology can be employed forthis purpose, provided that they can be used for the aforementionedcomponents or additional components.

It is advantageous for the construction of CT detectors, if the sensorchip is provided with laterally adjacent, in particular structurallyidentical, sensor chips for a flat arrangement. The advantage isparticularly great if the sensor chip can be arranged with additionalsensor chips making optimum use of the space on four sides and these canbe arranged on a flat plane or a cylindrical surface, as is usuallycustomary in CT detectors. Crucial to the shape of the sensor chip isthe element detecting radiation which should be a defined distance,ideally as short as possible, from each element detecting radiation ofan adjacent sensor chip. To this end, the size and lateral condition ofthe sensor chip can be adjusted in four directions accordingly. Idealshapes therefore appear to be prisms with a square base such as, forexample, a cuboid shape.

Computerized tomography detectors with a multiplicity of sensor chipsachieve a high resolution, even if each of the sensor chips onlycontributes one pixel to the medical image.

An embodiment of the invention is directed to a production method of asensor chip including at least:

a first step where components of an element detecting radiation andcomponents of an analog-digital converter are lithographicallyintegrated on a detector side of a crystalline base plate,

a second step where the detector side is insulated with the componentsof the element detecting radiation and the components of theanalog-digital converter,

a third step where a through-contact for the electrically conductiveconnection of conductor paths of the detector side is embodied withconductor paths of the second side of the crystalline base plate, and

a fourth step where the second side of the crystalline base plate isinsulated.

Finally, connection elements must still be applied on the second side asotherwise a connection to an analysis unit or computer would not bepossible.

In production, it is important that the insulation of the detector sideis removed from the base plate by the through-contact in a definedmanner in order to be able to make the through-contactelectroconductive. After the electrically conductive contact between thecomponents on the detector side and the components on the second sidehas been achieved through the base plate, final insulation of the secondside can take place.

An embodiment of the invention likewise comprises a method for theoperation of a sensor chip in which radiation is detected on onedetector side of a crystalline base plate of the sensor chip and thedetected radiation is converted into an analog signal and the analogsignal by way of an analog-digital converter into a digital signal.

As a result of the digital signal being conducted from the detector sidethrough a through-contact of the base plate to a second side of the baseplate, considerable wiring costs are saved. For CT detectors as well asfor other detectors which employ a multiplicity of sensor chips, thesavings result from a number of wires corresponding to the number ofpixels no longer needing to be produced in the conventional manner.Instead, the necessary contacts can be realized with little expense byway of semiconductor technology.

Advantageously, the digital signal on the second side of the base platecan be conducted by electrical or electronic components. For thisreason, the digital signal can still be processed on the second side ofthe base plate, for example, by additional components arranged there.Even if further processing is not to take place on the base plate, thedigital signal can be advantageously diverted from the sensor chip via aconnection element arranged on the second side of the base plate,without having to lead a cable around the sensor chip.

CT detectors, as used in X-ray computer tomographs, require amultiplicity of sensor elements which are arranged side by side oncylindrical or other surfaces in order to detect the X-rays directly orindirectly and convert them into a digital signal. Semiconductortechnology is used for the production of these sensor elements. Twoexample embodiments of the sensor chip according to the invention arepresented as sensor elements hereinafter by way of example.

FIG. 1 shows a sectional view of a first sensor chip in a productionsection before the through-contact. A detector side 12 of a base plate 1with the components arranged there was already insulated. Thesecomponents include a photodiode 9 and a section of an analog-digitalconverter 10, from which a conductor path 2 which is provided for athrough-contact emerges. The conductor path 2 is on a side of a layer 8facing the detector side 12 inside the sensor chip. When making thethrough-contact, the layer 8 on the conductor path 2 must be removed. Aconductor path 14 and a component 3 likewise constitute parts of theanalog-digital converter 10.

FIG. 2 shows the sensor chip from FIG. 1 after etching, insulation andmetal plating. Now the analog-digital converter 10 can pass on a digitalsignal via the conductor path 2 via the through-contact 6, in otherwords, also via the metal plating 5 to a conductor path 15 of theopposite second side 13 of the base plate 1 for further use.

The electrical contact is realized by the additional undersideapplication of the metallic through-contact 6 on the conductor path 2which is only conductive if the material of the base plate 1, forexample, silicon, was fully removed vertically by the etching process.In this case, the through-contact is also called “through Silicon via”(TSV). The signals which flow through TSV require a current of some 10pA (picoampere).

After the through-contact the insulation of the second side 13 and theapplication of the solder balls 7 can take place. These solder balls 7serve to transmit the digital signal to an insulated cable (not shown)and thus create an electrical connection to an analysis unit, forexample, a computer.

The detector side 12 may be defined as the upper side because there theradiation 11 to be detected by a scintillation element (not shown) or bya radiation source of the tomograph impinges on the sensor chip.Consequently, the second side 13 may be called the underside.

FIG. 3 shows a sectional view of the first sensor chip from FIG. 2 afterthe application of a connection element embodied as a solder ball 7.

An example embodiment of the first sensor chip of FIGS. 1 to 3 isadvantageous in that the components, namely the photodiode 9 and theanalog-digital converter 10 can be produced in a conventional CMOSprocess (CMOS=complementary metal-oxide semiconductor). The treatment oftwo base plates and their electrical connection is therefore completelyunnecessary. The potential for savings is considerable. Apart from thereduction in production costs, the robustness, service life andreliability of the sensor chip are also improved.

A large part of the space-saving advantage arises as a result of theoperating procedure of the first sensor chip of FIGS. 1 to 3 inparticular as a result of digital-analog conversion taking place in theimmediate vicinity of the element 9 detecting radiation, namely on thedetector side 12 of the base plate 1. Distribution of the digital signalvia the metal plating 5 of the through-contact 6 saves space as thedetector side 12 need not be equipped with cable connections which wouldhinder radiation detection. Instead, the second side 13, which isaverted from the radiation in any case, can be equipped with theconnection elements 7 or additional components.

Furthermore, there are advantages compared with the transmission ofanalog signals through the through-contacts, for the analog signalsreact sensitively to leakage currents in the picoampere range. This isnot the case with digital signals. Further robustness is achievedthereby.

FIG. 4 shows a cutaway view of a second sensor chip with aradiation-transmissive base plate 1 in which—unlike the first sensorchip of FIGS. 1 to 3—all the components are arranged on the detectorside 12.

A first advantage is that the photodiode 9 and the analog-digitalconverter 10 can be produced simultaneously in the same CMOS process.

A second advantage is that proximity of the analog-digital converter 10to the element 9 detecting radiation can be achieved without the use ofa through-contact. Instead the radiation is used for relativelyunproblematic transmission through the base plate 1. This exampleembodiment is a particularly cost-efficient solution for this reason. Itshould only be ensured that the material of the base plate 1 is asradiation-transmissive as possible for the radiation 11 to be detectedin order to achieve sufficient signal strength in the photodiode 9.

In summary, an embodiment of the invention relates to a sensor chip, inparticular for computerized tomography detectors, having ananalog-digital-converter 10 electrically connected to an element 9detecting radiation. The problem addressed by the invention is that ofdefining a sensor chip which is as cost-efficient and reliable aspossible. This is achieved by using one single crystalline base plate 1,on which all required components 7,9,10 of the sensor chip are applied,wherein a through-contact 6 between the conductor paths 14, 15 or thecontacts of both sides of the base plate 1 is used as applicable inorder to connect the components 7,9,10 of both sides 12, 13 to eachother.

1. A sensor chip, comprising: an analog-digital-converter electricallyconnected to an element to detect radiation and a crystalline baseplate, components of the element to detect radiation and components ofthe analog-digital-converter being lithographically integrated on onedetector side of the crystalline base plate.
 2. The sensor chip of claim1, wherein the analog-digital-converter is electrically connected to anelectrical connecting element on a second side of the crystalline baseplate, opposite the detector side of the crystalline base plate, via athrough-contact of the crystalline base plate.
 3. The sensor chip ofclaim 2, wherein the through-contact include metal plating, provided fortransmission of a digital signal from the detector side, through thecrystalline base plate, to the second side of the crystalline baseplate.
 4. The sensor chip of claim 3, wherein the through-contact isformed at least partially by a recess in the crystalline base plate. 5.The sensor chip of claim 4, wherein the electrical connecting element isapplied on the second side of the crystalline base plate.
 6. The sensorchip of claim 1, wherein the crystalline base plate is permeable for theradiation detectable by the element to detect radiation and theelectrical connecting element is applied on the detector side of thecrystalline base plate.
 7. The sensor chip of claim 1, wherein thecomponents of the element to detect radiation, the components of theanalog-digital-converter and the components of the connecting elementare electrical or electronic components.
 8. The sensor chip of claim 1,wherein the sensor chip is intended for flat arrangement with laterallyadjacent sensor chips.
 9. A computerized tomography detector comprising:a multiplicity of the sensor chips of claim
 8. 10. A production methodof a sensor chip, comprising: lithographically integrating, on onedetector side of a crystalline base plate, components of an element todetect radiation and components of an analog-digital-converter;insulating the detector side with the components of the element todetect radiation and the components of the analog-digital-converter;designing a through-contact, for the electrically conductive connectionof conductor paths of the detector side, with conductor paths of thesecond side of the crystalline base plate; and insulating the secondside of the crystalline base plate.
 11. A method for the operation of asensor chip, comprising: detecting radiation on one detector side of acrystalline base plate of the sensor chip; converting the detectedradiation into an analog signal; converting the analog signal into adigital signal by way of an analog-digital converter; and conducting thedigital signal from the detector side, via a through-contact of the baseplate, to a second side of the base plate.
 12. The method as claimed inclaim 11, wherein the digital signal on the second side of the baseplate is conducted by electrical or electronic components.
 13. Thesensor chip of claim 1, wherein the sensor chip is for a computerizedtomography detector.
 14. The sensor chip of claim 4, wherein the recessis an etched recess.
 15. The sensor chip of claim 7, wherein thecomponents of the element to detect radiation, the components of theanalog-digital-converter and the components of the connecting elementare undoped and/or doped semiconductor layers.
 16. The sensor chip ofclaim 8, wherein the sensor chip is intended for structurally identicalsensor chips.
 17. A computerized tomography detector comprising: amultiplicity of the sensor chips of claim
 1. 18. The method as claimedin claim 12, wherein the digital signal on the second side of the baseplate is conducted by components of a connecting element.
 19. The sensorchip of claim 2, wherein the crystalline base plate is permeable for theradiation detectable by the element to detect radiation and theelectrical connecting element is applied on the detector side of thecrystalline base plate.
 20. The sensor chip of claim 2, wherein thecomponents of the element to detect radiation, the components of theanalog-digital-converter and the components of the connecting elementare electrical or electronic components.